SARS-CoV Nucleocapsid Protein Antagonizes IFN-β Response by Targeting Initial Step of IFN-β Induction Pathway, and Its C-terminal Region is Critical for the Antagonism

Severe acute respiratory syndrome coronavirus (SARS-CoV) encodes a highly basic nucleocapsid (N) protein which can inhibit the synthesis of type I interferon (IFN), but the molecular mechanism of this antagonism remains to be identified. In this study, we demonstrated that the N protein of SARS-CoV could inhibit IFN-beta (IFN-β) induced by poly(I:C) or Sendai virus. However, we found that N protein could not inhibit IFN-β production induced by overexpression of downstream signaling molecules of two important IFN-β induction pathways, toll-like receptor 3 (TLR3)- and RIG-I-like receptors (RLR)-dependent pathways. These results indicate that SARS-CoV N protein targets the initial step, probably the cellular PRRs (pattern recognition receptors)-RNAs-recognition step in the innate immune pathways, to suppress IFN expression responses. In addition, co-immunoprecipitation assays revealed that N protein did not interact with RIG-I or MDA5. Further, an assay using truncated mutants revealed that the C-terminal domain of N protein was critical for its antagonism of IFN induction, and the N deletion mutant impaired for RNA-binding almost completely lost the IFN-β antagonist activity. These results contribute to our further understanding of the pathogenesis of SARS-CoV.